OF THE MORE DISTURBED ZONES OF THE EARTH'S CRUST. 445 



out a great portion of its length, this lofty and rugged zone of mountains consists 

 of two approximately parallel chief crests. The great feature in the geological struc- 

 ture of the whole zone is the presence of belts of closely plicated Mesozoic and Ter- 

 tiary strata on both flanks of each of these great constituent ranges. But the most 

 striking, and, at first view, perfectly enigmatical feature, is the inward plunge of 

 the newer strata beneath the older, in the sides and at the base of both chains. 

 When, however, the plicated strata are structurally arranged and traced, we find 

 that this phenomenon assumes the character of a symmetrical folding of the 

 rocks in two opposite directions from each high central axis. The individual 

 foldings, with scarce an exception, lean outwards from the central tracts of 

 the mountains, or from the quarters of igneous disturbance, rupture, and maxi- 

 mum metamorphism of the crust. In other words, the axis planes of the pli- 

 cated strata of the flanks of the Alps dip inwards towards the centre of the chain ; 

 those nearest to it at a low angle, and those more remote at angles steeper and 

 steeper as the waves recede, expanding to the outer base of the range. High on 

 the flanks of the Alps, or, what is the same thing, deep in towards the roots of the 

 mountain, where only the synclinal bends, of the flexed strata, have been protected 

 from denudation by inward folding, these closely compressed troughs lie pinched 

 in between the older strata in oblique inward inclination. The transverse sec- 

 tions expose these bendings, which are called Vs by some of the Swiss geologists. 

 Here then we behold an exact counterpart in the stratification or structure of a 

 single flank of the Alps, of that folding with inversion which characterizes the 

 Appalachian chain, or that of the Ardennes, a single side of the Alps being the 

 equivalent of the whole of either of those zones ; it consists, that is to say, of a belt 

 undulated in one direction. Crossing the Alps, or rather one of its component great 

 chains, we find another similar belt of the same strata, plicated in the same way, 

 with their axis planes dipping also under the crest or orographic axis of the 

 mountain, but of course, to the opposite quarter of the compass as compared with 

 the plicated zone of the other flank. This is, I conceive, a true picture of that 

 feature which, hitherto imperfectly analysed, has been called by some of the geo- 

 logists of Switzerland, expressively enough, 



The Fan-like Structure of the Alps. 



Viewed as a single chain, this mountain system consists, then, of two belts 

 undulating in opposite directions ; but, as already stated, it is for the most part of 

 its length a double chain ; and I think each range, especially, where these are 

 widest apart has a plicated belt of strata upon each of its slopes, so that, for some 

 districts at least, the fan-like structure is twice repeated ; in other words, there 

 are four belts of closely folded waves, each having its axis planes dipping to- 

 wards the base of its own high mountain system. 



